The present invention relates to electronically switched motors, also known as xe2x80x9cbrushlessxe2x80x9d motors, for applications which require high efficiency and low cost, simultaneously. A major although non-exclusive application for the invention lies in the automotive industry for driving actuators or servo-motor feed pumps with hydraulic liquid under pressure at flow rates that are very variable.
FIG. 1, shows an example of a conventional electronically switched motor. In the figure, the motor comprises a permanent magnet rotor 10 and a stator which carries three windings 12 and position sensors 18. A three-branch control module forming a bridge of switches serves to power the windings 12 from a direct current (DC) source 16. Each branch has two series-connected switches with general references 20 and 22, and individual references 20a-20c; 22a-22c, with the various branches being connected between a voltage source 16 and ground. Each switch is itself connected in parallel with a freewheel diode 24.
The circuit of FIG. 1 also has a subtracter 26 which has one input receiving a digital signal Vc giving a speed reference value and another input receiving a digital signal Vm which receives a digital signal representative of the speed of the rotor 10. The signal Vm is generated by a calculator 28 which receives the output signals from the position sensors 18.
The calculator 28 is designed to deliver a digital signal on its output 30 that is representative of the opening duty ratio or RCO of the periodic voltage pulses supplied to the windings 12 during successive periods of activation. This signal is generated on the basis of an error signal xcex5v supplied by the subtracter 26. A circuit 32 for controlling the power switches 20 and 22 generates switch closure signals on the basis of the value of the RCO and on the basis of the signals delivered by the sensors 18 which specify the position of the rotor, and also on the basis of a frequency control and synchronization signal 34.
Several ways of controlling such a circuit are already known, and in particular the so-called 120xc2x0 mode and the so-called 180xc2x0 mode.
In the 120xc2x0 type mode, the module feeds each phase of the motor in succession with periodic voltage pulses presenting a duty ratio that varies so as to regulate speed, but without any time overlap between periods during which power is applied to the various phases, there being successive periods in which the same phase is powered in one direction and then in an opposite direction, with these periods being separated by periods during which said phase receives no power.
That power supply mode presents the advantage of giving high efficiency. However high torque can be obtained only at low speed. 120xc2x0 type control provides an operating characteristic of torque as a function of speed which presents torque that is more or less constant, whereas it would be desirable to have a characteristic that approaches operation at constant power.
Once a duty ratio of 100% is reached, any increase in torque can be obtained only by increasing current, and current must itself be limited to a value which is compatible with the strength of the motor bearings.
In applications that require very high values of torque at high speeds of rotation, so-called xe2x80x9c180xc2x0 type modexe2x80x9d control is used. In that mode, the periods during which each switch is activated are such that all three phases are powered simultaneously, each phase being continuously powered in one direction or in the other. For this purpose, so-called 180xc2x0 type mode fills in the periods of no power feed that exists in 120xc2x0 type mode power supplies.
However, 180xc2x0 type mode presents reduced efficiency when the motor is lightly loaded, in particular because of the presence of a large reactive component which increases Joule effect losses.
The object of the present invention is to propose a synchronous electric motor that is controlled in such a manner as to present high torque at high speeds of rotation, and low losses at low speeds.
A second object of the invention is to propose a motor in which the control means do not give rise to any sudden changes of torque or of speed when switching between two modes, and to achieve this without requiring complex electronic control means.
According to the invention, this object is achieved by an electronically switched motor comprising a stator-rotor assembly fitted with three-phase driving windings and further comprising a series of switches for powering the windings and control means suitable for regulating the speed of the motor on a reference speed, said control means including means for monitoring the relative speed of the stator and the rotor, and means for causing each switch to switch ON and OFF in compliance with a repeated sequence selected to power the driving windings as a function of the signals supplied by the speed monitoring means, said sequence including a succession of ON/OFF pulses with a duty ratio that is a function of the speed monitoring, the motor being characterized in that the control means are suitable for responding to signals delivered by the monitoring means to generate two modes for controlling the switches, the sequence of the second switch control mode having switch ON states that include the OFF states of the switches in final operation of the first mode, and an ON state or a succession of ON pulses in phase advance which is directly extended by one of said ON states.
Such a motor is particularly advantageous because it requires no intensive calculation to smooth the transition between modes.